Fuses are employed in integrated circuit devices for the purpose of removing defective circuits from active operation and for replacing those defective circuits with good operable redundant circuits. Typically the fuses are fabricated in a conductive layer buried within a structure of surrounding insulator material. The fuses are fabricated in either polysilicon or metal. Typically the insulator is silicon dioxide. The top surface of the fuses is near the top surface of the insulator but is covered by some of the insulator material. Other conductive layers are formed over the top of the insulator material covering the fuses.
A selected fuse is blown, or trimmed, by directing a laser beam at the fuse. Laser energy is absorbed by the fuse material and becomes thermal energy. The fuse material is changed from its original solid state to a liquid state in response to the thermal energy absorbed from the laser beam. As the temperature of the fuse material increases above its boiling point, internal pressure builds up inside of the device structure at the region of the fuse where the laser beam is directed. Pressure exerted by the insulator on the fuse, temporarily maintains the fuse material in its liquid state. As the temperature of the liquid rises, the internal pressure rises higher and higher until stress in the surrounding insulator exceeds the fracture limit of the insulator. The insulator breaks at the weakest point which is toward the top surface of the insulator.
Instantly when the insulator breaks, the pressure on the liquid fuse material drops to ambient pressure and the volume of the liquid fuse expands explosively. Because the temperature of the fuse material is above its boiling point and is at atmospheric pressure, the liquid fuse material vaporizes instantly. The fuse material is blown up out of the insulator leaving a section, which had been occupied by conductive fuse material, completely void of conductive material. Thus an open circuit remains.
When a fuse is blown, the shape of the dielectric is important so that the direction of the explosion is upward from the top surface of the device. If the explosion occurs through the top dielectric covering the fuse, vaporization of the fuse material occurs smoothly because of the large open space above. On the other hand, an explosion to the side causes problems.
As shown in FIG. 1a, in some integrated circuits fuses 10 and 11 are fabricated in the top lead layer of conductors of the integrated circuit device 12. Such top lead layer fuses 10 and 11 are covered by a layer of cap oxide 15 which covers both the top surface and the sidewalls of the fuse material 10, as shown in FIG. 1b.
Typically the dielectric is a cap oxide deposition that proceeds until the dielectric is 3150 .ANG.+/-150 .ANG. thick. The deposition is made at a temperature of 250.degree. C.+/-10.degree. C. and at a pressure of 2.5 TORR+/-0.2 TORR. Gas flow is SiH.sub.4 : 40 SCCM+/-5 SCCM and N.sub.2 O: 1300 SCCM+/-100 SCCM. Typically the cap oxide is deposited thicker on the top surface of the fuses than on their sidewalls, as is readily apparent in FIG. 1b. Although the cap oxide insulates the fuses very well, it becomes a source of a problem when the fuses are to be selectively blown, or trimmed.
When a laser beam 18 is directed at the fuse 10, as shown in FIG. 1c, that fuse liquifies, and the cap oxide fractures at the weakest place 20. The weakest place is the thin sidewall rather that the thicker cap oxide 22 over the top of the fuse 10. As shown pictorially in FIG. 1d, the vaporized fuse material 24 is released out the sidewall of the structure and toward the next fuse 11. Slag material 26 from the blown fuse spreads around onto the cap oxide surface between the fuses 10 and 11 and onto the next fuse structure. Depending upon the pattern of slag and the selection of fuses to be blown, undesirable conductive paths can be created through the scattered slag.
Rather than repairing a defective device, the device 12 continues to be defective but with a different defect.